Gravimetric Analysis Flashcards

Flashcards about Gravimetric Analysis

What is Gravimetric Analysis?

Isolating and weighing an element or a definite compound of the element in as pure a form as possible.

What are the two types of Gravimetric methods?

Precipitation and volatilization methods.

What happens in Precipitation methods?

The analyte is converted to a sparingly soluble precipitate that is filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, then weighed.

What happens in Volatilization methods?

The analyte or its decomposition products are volatized at a suitable temperature, then the volatile product is collected and weighed.

What are the advantages of gravimetric analysis?

Accurate and precise data, readily checked sources of error, an absolute method, and relatively inexpensive apparatus.

What operations in a gravimetric procedure require little attention?

Drying, igniting, digesting, and evaporating.

Which other analytical method does not require calibration or standardization?

Coulometry.

What limits the sensitivity and accuracy of a gravimetric method?

Solubility losses, co-precipitation errors, and mechanical losses of precipitate.

What concentration should rule out a gravimetric method?

Below 0.1%.

Are gravimetric reagents specific?

They are selective in the sense that they tend to form precipitates with groups of ions.

Why do gravimetric methods not require calibration?

Gravimetric methods do not require a calibration or standardization step because the results are calculated directly from the experimental data and atomic weights.

What is the equipment required for a gravimetric analysis like?

Simple, relatively inexpensive, reliable, and easy to maintain.

For what have gravimetric methods been developed?

Inorganic anions and cations, as well as for neutral species as water, sulfur dioxide, carbon dioxide and iodine.

What do common inorganic precipitating agents typically form with the analyte?

Slightly soluble salts or hydrous oxides.

What two types of products do organic reagents form?

Coordination compounds (nonionic) and products in which the bonding between the inorganic species and the reagent is largely ionic.

What are reagents that form sparingly soluble coordination compounds called?

Chelating agents, chelates.

What are the solubilities of metal chelates?

Low in water but high in organic liquids.

Name some organic functional groups suitable for gravimetric methods.

Aromatic carbonyl, methoxyl and ethoxyl, aromatic nitro, azo, phosphate, sulfamic acid, sulfinic acid.

What is oxine sometimes called?

8-Hydroxyquinoline.

Which chelate has a beautiful and vivid red color?

Nickel dimethylglyoxime.

What are the solubilities of metal 8-hydroxyquinolates like?

Vary widely from cation to cation and are pH-dependent.

What does Dimethylglyoxime precipitate from a weakly alkaline solution?

Only nickel(II).

What is Creeping?

The process by which a precipitate moves up the sides of a wetted surface of a glass container or a filter paper.

In what solutions is Sodium tetraphenylboron a near-specific precipitating agent for potassium and ammonium ions?

Cold mineral-acid solutions.

What are the three steps in relating reactant weight to product weight?

Transformation of weight to moles, multiplication by a stoichiometric factor, reconversion to metric units.

From what two experimental measurements are the results of a gravimetric analysis computed?

The weight of sample and the weight of a product of known composition.

What is the equation for calculating percent analyte?

weight of A / weight of sample X 100%.

How is the weight of analyte obtained?

Multiply the weight of the final product by a constant made up of conversion factors and the stoichiometric relationship.

What is the gravimetric factor (GF)?

A constant made up of conversion factors and the stoichiometric relationship between the analyte and the product weighed.

What is a general definition for the gravimetric factor?

(a/b) * (fw analyte / fw product).

What is the mass percent of a constituent?

Mass of constituent / Mass of sample! X 100.

What is a general equation for gravimetric calculation?

Mass of precipitate X gravimetric factor X 100 / Mass of sample.

What is an alternate name for Gravimetric Analysis?

Quantitative analysis by weight

What is the name of the instrument that yields highly accurate and precise data, based upon mass measurements?

Analytical balance

What can filtrates be tested for?

completeness of precipitation

What may precipitates be examined for?

presence of impurities

What kind of measurement does the absolute method involve?

direct measurement

What is the most expensive requirements for determinations?

muffle furnace

All other analytical methods require preparation of standard solutions with the exception of what?

coulometry

A gravimetric method requires only the calculation of a gravimetric factor from data in a table of __.

atomic weights

With a suitable balance it is perfectly feasible to obtain the weight of __ of material to within a few parts per thousand of Its true value

a few micrograms

For __, gravimetric procedures are seldom surpassed in accuracy

simple samples containing more than 1% of the analyte

Gravimetric reagents are __ but are instead selective

seldom specific

Gravimetric reagents tend to form precipitates with __

groups of ions

When only one or two samples are to be analyzed, a gravimetric approach may be the method of choice because it __ than a procedure that requires preparation of standards and calibration

requires less time and effort

The equipment required for a gravimetric analysis is __

simple, relatively Inexpensive, reliable and easy to maintain

These reagents typically form __ salts or hydrous oxides with the analyte.

slightly soluble

One type of organic reagents forms slightly soluble, nonionic products called __

coordination compounds

Reagents that form compounds that bond with a cation by sharing a pair of electrons are called __

chelating agents

RCHO + H2NNHCH3(NO2)2 = R-CH NNHC6H3(NO2)2(s) + __

H2O

CHI+Ag + H₂O → __ + CH3OH

AgI

What is a salt rather than a chelate?

Potassium tetraphenyi boron

What is the reaction for silver chloride precipitation?

This equation, Ag^+ + Cl^- \rightarrow AgCl(s), represents the precipitation of silver chloride when silver ions react with chloride ions in solution. It's a classic example in gravimetric analysis.

What is the equation for the Gravimetric Factor (GF)?

The gravimetric factor (GF) is defined as: GF = \frac{molar \, mass \, of \, analyte}{molar \, mass \, of \, precipitate} \times \frac{a}{b}, where a and b are stoichiometric coefficients.

How do you calculate the mass of analyte using the gravimetric factor?

To calculate the mass of analyte from the mass of precipitate: Mass \, of \, analyte = Mass \, of \, precipitate \times GF

What is the formula for calculating % Cl using AgCl precipitate?

The formula for calculating the percentage of chloride in a sample using silver chloride precipitation is: % Cl = \frac{Mass \, of \, AgCl \times GF \times 100}{Mass \, of \, Sample}, where GF = (atomic mass of Cl) / (molar mass of AgCl).

What is the reaction for determining sulfate as barium sulfate?

The reaction for determining sulfate as barium sulfate is: Ba^{2+}(aq) + SO4^{2-}(aq) \rightarrow BaSO4(s).
Barium sulfate is precipitated from a solution containing sulfate ions by adding barium chloride.

What is the formula for calculating % SO4^{2-} using BaSO4 precipitate?

To find the percentage of sulfate (SO4^{2-}) in a sample: % SO4^{2-} = \frac{Mass \, of \, BaSO4 \times GF \times 100}{Mass \, of \, Sample}, Where GF = (molar mass of SO4^{2-}) / (molar mass of BaSO_4).

What is the relationship between moles of analyte and precipitate?

The relationship between the moles of analyte and precipitate is essential for calculation: moles \, of \, analyte = moles \, of \, precipitate \times stoichiometric \, factor

How do you correct for moisture in a precipitate?

To correct for moisture in the precipitate, dry the precipitate in a drying oven, let it cool in a desiccator, and then weigh. The formula is Corrected \, Weight= Original \, Weight - Weight \, of \, water

Example: Calculating %Cl from AgCl precipitate.

If 0.5000 g sample yields 0.4000 g of AgCl, the %Cl is calculated as %Cl = (0.400 \, g \, AgCl \times 35.45 \, g/mol) / (143.32 \, g/mol \times 0.5000 \, g) \times 100 = 19.78 \,%

Example: Calculating %SO4^{2-} from BaSO4 precipitate.

If 0.6000 g sample yields 0.3000 g of BaSO4, the %SO4^{2-} is calculated as: %SO4^{2-} = (0.300 \, g \, BaSO4 \times 96.06 \, g/mol) / (233.39 \, g/mol \times 0.6000 \, g) \times 100 = 20.57 \,%

How to find the percentage of water in hydrated salt?

The percentage of water in a hydrated salt can be determined using the formula: %H_2O = \frac{Mass \, of \, Water \, Lost}{Mass \, of \, Hydrated \, Salt} \times 100

Example: Percentage of Water in Hydrated Salt

If 1.000 g of hydrated salt loses 0.200 g of water upon heating, the %H_2O is: \frac{0.200 \, g}{1.000 \, g} \times 100 = 20.0\,%

What is the reaction for determining iron as iron oxide?

The reaction for determining iron as iron oxide is: 2Fe^{3+}(aq) + 3O^{2-}(aq) \rightarrow Fe2O3(s). Ferric ions are precipitated as iron oxide (Fe2O3) after ignition.

How do you calculate the gravimetric factor for Fe2O3?

The gravimetric factor for Fe2O3 is calculated as: GF = \frac{2 \times Atomic \, Weight \, of \, Fe}{Molar \, Weight \, of \, Fe2O3}. This is because 2 moles of iron produce 1 mole of iron oxide.

Example: Calculating %Fe from Fe2O3 precipitate.

Given a sample of 0.800 g yields 0.250 g of Fe2O3, the %Fe is: %Fe = \frac{0.250 \, g \times (2 \times 55.845 \, g/mol)}{159.69 \, g/mol \times 0.800 \, g} \times 100 = 21.84 \,%

Direct Precipitation Reaction

In direct precipitation: Analyte(aq) + Reagent(aq) \rightarrow Precipitate(s). This involves the direct addition of a reagent to precipitate the analyte.

Explain Volatilization Methods

In volatilization methods, the analyte is converted to a volatile form, which is then collected and weighed. For example, determining carbon dioxide by absorbing it in a suitable absorbent.

What is Combustion Analysis?

Combustion analysis involves burning an organic compound and measuring the amounts of CO2 and H2O produced. The empirical formula can then be determined.

Combustion Calculation: % Carbon

For CxHy + O2 \rightarrow xCO2 + \frac{y}{2}H2O, the formula to calculate mass percent of carbon: %C = \frac{mass \, of \, CO2 \times \frac{12.01}{44.01}}{mass \, of \, sample} \times 100

Combustion Calculation: % Hydrogen

For CxHy + O2 \rightarrow xCO2 + \frac{y}{2}H2O, the formula to calculate the mass percent of hydrogen: %H = \frac{mass \, of \, H2O \times \frac{2.016}{18.015}}{mass \, of \, sample} \times 100

What is the Solubility Product Expression?

Used to predict if a precipitate will form: If Q > K{sp}, precipitation occurs until Q = K{sp}.

What

The solubility product (K{sp}) is the equilibrium constant for the dissolution of a solid in a liquid. For AgCl, K{sp} = [Ag^+][Cl^-].